I was--and of course still am-- a native speaker of Standard American English (SAE), born and raised in the San Francisco Bay Area with a master's degree in history from San Francisco State University. I'd lectured in community colleges and published newspaper articles before I flunked an English-language proficiency exam.  As a result, I was deemed unqualified to Teach English as a Foreign Language (TEFL, not to be confused with TESL, which is Teaching  Englsih as a Second Language, a distinction very important to career professionals, by their account) in an educational center in Bogota, Colombia whose stated mission was to teach young adult Colombians to speak and understand  "English as it is spoken by well-educated native speakers in North America today."

I thought that was right up my alley. but my English weren't gud enuff, they done  tole me.

Granted, I came close, scoring 78%  when the minimum passing grade was 80%. El Centro Colombo-Americano, something called a "bi-national center" financed by the US State Department, housed in its own well-appointed six-story building in downtown Bogota,  would hire many  non-native speakers but no one who flunked their written test. When you failed their exam you could not retake it for six months.

Shocking, yes.  Mortifying, yes.  In fact, humiliating is the word.  My girlfriend/partner/compaion passed the exam easily and was quickly hired full-time. We had entered Colombia on tourist visas which would soon expire. I was suddenly desperate for a job, viable options were limited, and I ended up having to take a job at a shady language school called Instituto Meyer, the kind of "school" that advertised on matchbook covers and had a sleazy reputation for poor education and recurring financial fraud. They kept dissolving in bankruptcy and resurfacing under another name.  The students didn't learn much and the teachers often didn'tget paid, or were paid half of what they were owed, several weeks late. If those teachers quit they lost their visa ordinaria, which allowed them legally work and live in Colombia, because their  year-long work contract with a specific school was the basis for that visa. You suddenly became an "illegal" and faced deportation if you quit your shitty job because you hadn't been paid for several months. Working there was where I learned the term "cheque chimbo" which means a "bad check." You might call it the "Chinatown sweatshop  syndrome" in reverse. Instituto Meyer was owned and managed by a man who called himself  "Doctor"  Harmon Meyer and announced he was "born and raised in Puget Sound, Washington"---except he had a thick West Indian accent and pronounced "Puget" to rhyme with "budget." He claimed ot have doctoral degrees in several unrelated fields and to have taught at UCLA, London School of Economics, the Sorbonne, etc. , "while publishing numerous scholarly  books and articles."  Eventually I was told his real name--supposedly his real name--was Armando Flores and he was from San Andres Island and had no degrees from anywhere.  I felt ashamed to tell people where I worked. His "school" (the quotation marks are deserved) had very poorly-designed and poorly-written, sloppily-organized texts, but I met some interesting colleagues there, I put in my six months, did my best for the students, learned some ESL skills, and when  I retook the Colombo-Americano test, I passed with flying colors, scoring 98% instead of 78%.

How had I flunked the first time? By not knowing what "pluperfect" meant or what a "subjunctive tense" was. I read a lot as a child and I learned what I know about grammar, verb tenses, sentence construction, spelling, punctuation, and capitalization by osmosis, as it were, like  someone who learned to play music by ear. I never consciously learned the implicit rules. I could never identify the "parts of speech" or diagram sentences correctly. I hated being assigned to diagram sentences. (Do they still do that in school?)

Our  scored tests were never returned to us to allow us to review our errors  but I do  remember one section  that left me completely baffled and hopelessly lost. The instructions were to look at the following twelve sentences, then underline any subordinate clauses in each of them, then identify those clauses as either dependent or independent clauses, and identify them as adjectival clauses, noun clauses, or adverbial clauses. I had no idea what they were talking about.  I'm sure I flunked that section.

Once I did start teaching, some students would stump me with questions such as "Is it true that any time you have a verb in the quantum-flux  super-pluperfect-subjunctive tense, you must also have..." and I soon learned --in self-defense--to interrrupt them by asking for an example, which I would then support or correct.

"Teacher, can I say, 'my father he goes to the work all of the days'?

"It's better to say, 'my father works every day' or 'my father goes to work every day' but don't ask me why."

Language is, after all, a matter of custom and tradition that is continuously evolving. I actually learned a lot about English by teaching it--but at the time, flunking that test of proficiency in my own native language after earning two university degrees was, yes,  mortifying.